1. Field of the Invention
This invention relates generally to nuclear reactor fuel element support grids and more particularly to support grids formed from lattice straps which are metallurgically bonded to a perimeter strap.
2. Background Information
Nuclear fuel assembly grids are used to precisely maintain the spacing between fuel rods in a nuclear reactor core, prevent rod vibration, and provide lateral support for the fuel rods. Grids are made of materials with low neutron absorption cross-sections such as stainless steel, inconel, and alloys of zirconium to minimize grid deformation and the loss of structural integrity during irradiation. Conventional designs of grids for nuclear reactor fuel assemblies include a multiplicity of interleaved inner grid straps formed in an egg-crate configuration defining cells which accept fuel rods. The ends of each of the inner grid straps are interlocked with an outer grid strap, forming the peripheral cells of the grid. Each cell provides support to one fuel rod at a given axial location through the use of relatively resilient springs of various forms. In order to minimize the lateral displacement of fuel rods and to improve the fuel characteristics of an assembly, a number of grids are spaced along the fuel assembly length. In pressurized water reactors, typically each grid is held in place along the fuel assembly by attachment to control rod guide thimbles. In boiling water reactors, typically a can surrounds the fuel rods and grids. In the boiling water reactor, each grid is held in place along the fuel assembly length by a special locking rod.
The interior straps that are interlocked in an egg-crate pattern are generally held in place by a welded or brazed joint at their intersecting locations. The ends of the lattice straps are similarly affixed to a perimeter strap that surrounds them by welds or brazes. If the straps are made of Zircalloy or stainless steel, they can generally be welded. If inconel or nickel-plated inconel are employed, they generally have to be brazed.
Designers are constantly seeking to improve the means of manufacture of the grids. Areas of interest include mechanisms for reducing the manufacturing effort, and meeting the stringent design envelope, or tolerances, on dimensional parameters of the grid. Further considerations include retaining the structural rigidity of the grid, and reducing the amount of material in the grid for minimizing neutron absorption and coolant flow interference by the grid. More particularly, a need specifically exists for an improved means for connecting the outer grid strap to the interior straps.
The peripheral straps are typically provided with anti-snag tabs that protrude from the top and bottom of the strap at spaced locations and angle slightly towards the interior of the grid. The anti-snag tabs prevent the grids from hanging up on adjacent fuel assemblies as they are loaded or unloaded into or out of the core of the reactor. It has been found that the rubbing or bumping of the grids at the anti-snag locations has placed increased strain on the metallurgical joints that bond the interior grid straps to the peripheral strap. This has proved to be a particular problem in brazed joints causing some of them to fail. Accordingly, a further need exists for an improved means for connecting the outer grid strap to the interior straps.